The present invention is related generally to the field of locating and/or guiding an underground boring tool using a locating signal which is transmitted through the ground and, more particularly, to a method and associated apparatus for locating and/or guiding the boring tool in a way which compensates for skin effect that potentially introduces error in locating and/or guiding the boring tool as a result of conductivity of the earth through which the locating signal passes. A multi-frequency locating system is introduced including highly advantageous transmitter and locator configurations. A highly advantageous tone detection arrangement is also introduced.
Referring to FIG. 1, boring tools are typically guided or located by transmitting a dipole field from a dipole transmitter which is positioned within the drill head of the boring tool. The locating/dipole field is an oscillating signal that is generally emitted from a dipole antenna oriented along the rotational axis of the drill head. FIG. 1 illustrates a coordinate system including x, y and z axes with a dipole transmitter D at its origin. For a point p, at a radius r from the origin, the dipole equations are given as:                               B          x                =                                            3              ⁢                              x                2                                      -                          r              2                                            r            5                                              (        1        )                                          B          y                =                              3            ⁢            xy                                r            5                                              (        2        )                                                      B            z                    =                                    3              ⁢              xz                                      r              5                                      ,                  xe2x80x83                ⁢        and                            (        3        )                                          r          2                =                              x            2                    +                      y            2                    +                      z            2                                              (        4        )            
Where Bx, By and Bz represent orthogonal components of the dipole field at point p. The dipole equations are recited herein for the benefit of the reader since these equations form a fundamental basis for the use of a dipole field in locating applications. One such locating system is described, for example, in U.S. Pat. No. 5,337,002 which is commonly assigned with the present application. Traditionally, boring tool systems have not used compensation for conductivity of the soil even though this conductivity introduces a phenomenon commonly referred to as skin effect. While skin effect can result in significant locating errors, applicants submit that prior art systems have not provided such compensation, at least in part, since it is perceived in the art that compensation for skin effect is an extremely complex proposition.
What prior art system designers have generally done is to altogether ignore skin effect. This is tantamount to an assumption of a non-conducting earth. Accordingly, the electromagnetic field emitted by the magnetic dipole of a transmitter into a non-conducting medium (such as air) is described mathematically by the well known cubic law of a magnetic dipole (see FIG. 1). Unfortunately, however, as a direct result of skin depth, drilling in the earth can produce significant deviations from the cubic law when a typical oscillating magnetic dipole field is used. The latter term describes a magnetic dipole having a signal strength that varies sinusoidally with time.
The present invention provides a highly advantageous and heretofore unseen method and associated apparatus which provide compensation for skin effect in underground boring tool applications.
As will be described in more detail hereinafter, there are disclosed herein arrangements, apparatus and associated methods for skin depth compensation in underground boring applications. Accordingly, in an overall method of operating a system in which a boring tool is moved through the ground in a region which includes an electrical conductivity characteristic and where the system includes an above ground arrangement for tracking the position of and/or guiding the boring tool as the boring tool moves through the ground and in which the system is configured for transmitting a locating signal between the boring tool and the arrangement in the region, the improvement comprises compensating for skin depth error by measuring the locating signal such that measurements of the locating signal include skin depth error introduced as a result of the electrical conductivity characteristic and, thereafter, using the measurements in a way which determines a skin depth corrected position of the boring tool.
In one aspect of the invention a multi-frequency approach is provided which utilizes measured intensities of the locating field at two or more frequencies to extrapolate a zero frequency value of locating signal intensity. The zero frequency value of intensity is then used in position determination. The multi-frequency approach may be used in conjunction with walk-over type locators or with one or more above ground receivers designed for receiving the locating signal at fixed position(s). In one feature, the multi-frequency approach of the present invention does not require knowledge of earth properties or ground surface geometry. The components of the measured magnetic field intensities of the locating field measured at their selected frequencies contain property and geometry effects and pass them on to extrapolated zero frequency values.
In another aspect of the invention, certain intensity measurements of the locating signal are used to determine a value for skin depth to be used during subsequent drilling, these certain measurements being obtained in a calibration procedure by transmitting the locating signal from the boring tool on the surface of the ground to the above ground arrangement prior to drilling.
In still another aspect of the invention, a determined value of skin depth is used in one locating scenario with a walkover detector in which the walkover detector is used to establish an overhead position directly above the boring tool using a locating signal transmitted at a single frequency. The measured overhead signal strength of the locating signal transmitted from the boring tool is then used in conjunction with the determined value of the skin depth to determine the depth of the boring tool below the overhead position on the surface of the ground such that the depth of the boring tool is established based at least in part on the skin depth.
In another locating scenario, with the locating signal transmitted at a single frequency, the boring tool moves through the ground along an intended path while transmitting the locating signal and moves in an orientation which includes pitch. The boring tool includes pitch sensing means and the locating signal exhibits a field defined forward point at the surface of the ground with the boring tool at a particular point along the intended path. The field defined forward point being vertically above an inground forward point on the intended path through which the boring tool is likely to pass. The boring tool is located by using a walkover detector to receive electromagnetic data which identifies the forward point. Signal strength of the locating signal is then measured at the forward point, as transmitted from the boring tool at the particular point, and the measured signal strength of the locating signal is used at the forward point in conjunction with the determined value of the skin depth and a sensed pitch value to determine the depth of the boring tool referenced to the particular point and to determine a forward distance on the intended path from the particular point at which the boring tool is located to the in-ground forward point.
Alternatively, the field defined forward point may be located on or immediately above the surface of the ground and an overhead point may be identified on or immediately above the surface of the ground directly above the boring tool at the particular point. The forward distance is measured between the overhead point and the forward point as, defined at the surface of the ground. Using the forward distance, the determined value of skin depth and certain characteristics of the locating signal at the forward point, a skin depth corrected depth of the boring tool at the particular point is determined.
In another alternative, the intended path of the boring tool in the region is configured such that the forward point is at a higher elevation on the surface of the ground than the particular point. The actual depth of the boring tool is then established at the particular point and a vertical elevation difference is measured between the particular point and the forward point. Thereafter, the locating signal is sensed at the forward point while the boring tool is at the particular point to determine an uncorrected depth of the boring tool which is subject to skin depth error. Using the measured vertical elevation difference, the actual depth of the boring tool at the particular point and the uncorrected depth of the boring tool measured from the forward point, a forward point skin depth correction factor is determined. During subsequent drilling operations the forward point skin depth correction factor is used in determining skin depth corrected depth with the boring tool at subsequent particular points.
In another aspect of the invention, using a system in which a boring tool is moved underground in a region during selective rotation of the boring tool, the boring tool is configured with a transmitter for transmitting a locating signal for use in tracking an underground position of the boring tool in the region and for changing at least one characteristic of the locating signal responsive to subjecting the boring tool to a predetermined roll sequence during underground operation. The predetermined roll sequence includes the steps of rotating the boring tool for one time period at a first roll rate in timed relation to rotating the boring tool for another time period at a second roll rate, followed by a halt in rotation. In one feature, the characteristic is the frequency of the locating signal. In another feature, the characteristic is the power of the locating signal.
In still another aspect of the invention, a transmitter, configured for installation in a boring tool, includes a first arrangement for transmitting a locating signal at a selected one of at least two frequencies. A frequency selection arrangement, cooperating with the first arrangement, determines the selected one of the frequencies based, at least in part, on a pitch orientation of the transmitter. In one feature, the frequency selection arrangement determines the selected one of the frequencies responsive to the pitch orientation of the transmitter upon power-up of the transmitter. In another feature, the frequency selection arrangement is configured for detecting a pitch orientation sequence to which the transmitter is subjected. Responsive to the detected pitch orientation sequence, the locating signal frequency is changed.
In yet another aspect of the invention, a transmitter configured for installation in a boring tool includes a first arrangement for transmitting a locating signal having a selected one of at least two frequencies for use in tracking the boring tool and a frequency selection arrangement, cooperating with the first arrangement, for detecting the selected one of the frequencies as a power-down frequency at a time when the transmitter is switched from an operational state to an off state and for restarting the first arrangement at the power-down frequency upon switching from the off state to the operational state.
In a further aspect of the invention, a locator for receiving the locating signal is configured for receiving the locating signal at any selected one of the locating frequencies for use in tracking the boring tool. A control arrangement, forming part of the locator, detects the selected one of the frequencies as a power-down locating frequency at a time when the locator is powered down and, thereafter, powers up the locator at least initially configured for receiving the power-down locating frequency.
In another aspect of the invention, the system includes a locator for tracking the position of and/or guiding the boring tool as the boring tool moves through the ground. The boring tool includes a transmitter which transmits a locating signal at a selected one of at least two frequencies for use in tracking the boring tool. The locator, in turn, receives the locating signal. The selected frequency of the locating signal is indicated to the locator by the boring tool using a frequency control arrangement which forms part of the transmitter. In one implementation, a frequency indication is encoded on a carrier, which carrier is also used in determining the depth of the boring tool. In another implementation, the frequency indication is encoded on a carrier which is distinct from another carrier that is used in the boring tool depth determination.
In still another aspect of the invention, in a system in which a boring tool is moved through the ground in a region including a locating arrangement for tracking the position of and/or guiding the boring tool as the boring tool moves through the ground, the locating arrangement includes a transmitter forming part of the boring tool for transmitting a locating signal at a selected one of at least two locating frequencies and for transmitting a frequency designation identifying the selected locating frequency of the locating signal. A locator is included in the system configured for receiving the locating signal for use in tracking the boring tool and itself including a frequency tracking arrangement for switching the locator between different ones of the locating frequencies based on the frequency designation.
In a further aspect of the invention, a tone decoder is disclosed for decoding an incoming analog data stream containing at least one tone. The incoming analog data stream is converted to a binary data stream based on one switching threshold. The binary data stream is then sampled over a sample period to establish a plurality of samples, each of which is characterized as a binary value, at a rate based on the tone. The samples are used in a way which establishes at least an approximate magnitude of the tone for the sample period. In one feature, alternating ones of the samples are used in contributing to a first value and a second value such that the first value and the second value are cooperatively indicative of at least the approximate magnitude of the tone.
In another aspect of the invention, a tone detection arrangement is disclosed for decoding an incoming data stream that is received in sequential bit times and which incoming data stream contains at least one tone that is selectively present for the duration of each bit time. A plurality of digital filters forms part of the detection arrangement, each of which is tuned for detecting the tone over a filter interval that is at least approximately equal in duration to the bit time from a filter start time to a filter stop time. A first one of the digital filters is started at a first start time in relation to a particular bit time. An additional one of the digital filters is started at an additional start time which occurs following a predetermined interval after said start time of the first digital filter such that a number of the predetermined intervals at least approximately equals the bit time in duration. At the filter stop time of the first digital filter, at least an approximate magnitude of the tone is determined for the filter interval of the first digital filter. At the filter stop time of the additional digital filter, at least the approximate magnitude of the tone is determined for the additional filter interval of the additional digital filter. In one feature, the filters are successively started and restarted in a staggered timed relation such that one filter outputs the approximate magnitude of the tone at a repeating interval corresponding to the predetermined interval at which the filters are started and restarted.
In yet another aspect of the invention, a tone detection arrangement decodes an incoming data stream which contains at least one tone that is selectively present. The tone detection arrangement includes a plurality of digital filters each of which is tuned for detecting the tone over a filter interval from a filter start time to a filter stop time. The digital filters are started in a staggered time relation with respect to one another so as to operate over a plurality of intervals that are in said staggered time relation with respect to one another including a plurality of said filter stop times which conclude the filter intervals in the staggered time relationship. An average magnitude of the tone is determined responsive to the filter stop time of each filter.